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2 sneet-sheet 1 Sept. 13, 1932. G. lslNG GALVANOMETER Filed oct. 1e.1929 Sept. 13, 1932. G. lslNG 1,877,243

GALVANOMETER Filed tOct. 116, 1929 2 Sheets-Sheet 2 Patented Sept. 13,1932 dUNE'ED STAT-ES PATENT ortica 11.

GUSTAF ISING, F STOCKS'UND, 'SWEDEN GALVANOMETER Application filedOctober 16, 1929, Serial N'o, 399,991, `and in Sweden October 19, 1928.

The present invention relatesto moving-l which the said conditions areall simultane-y ously complied with.

As is well known, there has existed a certain definite limit above whichthe sensitivity lof a galvanometer could not be increased. y Said limitwas determined by the occurrence of such irregular. oscillations of themovable system about its position of equilibrium asl are caused eitherby shocks of the molecules of the surrounding medium or by small. ir-

:regular current fluctuations of molecular v origin in the closedcircuit. From the equipartition principle, it follows that the meanamplitude of the resulting Brownian movement of the coil will be thesame, whatever Y. be the cause of this motion.

According to the invention, the maximum theoretical limit of thesensitivityof a galvanometer is in eect reached, on the one hand, byincreasing the optical accuracy to such 'extent as to make the saidBrownian Cil fluctuations visible; and on the other hand, by

choosing a theoretically favorable value ofv the damping of the movablesystem; and by removing` the influence of external disturb# ances andconcussions. The ideal conditions contemplated by the present inventionare as Y follows: Y i (a) The magnification of the deflection producedby optical means should be so high,

i that the mean value of the Brownian fluctuations corresponds to asmall fraction, for instance 1/15-1/10, of a division on the readingofi'scale, which scale is itself just discernible by the naked eye. (t) Inorder to reduce the apparent current fluctuations, which correspond tothe Brownian vibrations of the coil, as nearly as, possible at yfixedvalues of the time required for the deflection and at fixed values oftheresistance of the coil circuit, the electromag- Vnetical dampingshouldnot be made too small.

If the so-called critical damping (which corresponds to the aperodicallimit) is chosen as unit, the said electro-magnetical damping should notbe appreciably smaller than unity and should preferably considerablyexceed the air damping. y'

(c) The instrument should be free from external disturbances to suchextent that the temporary displacements of the Zero position, caused bysuch external influences are, on an average, smaller than the mean Valueof the Y Brownian fluctuations.

In order to embody the condition (a) the coil is in one form providedwith `a light pointer, consisting, e. g., of a fine quartz or glasswire, the end of which is observed underv a very high microscopicmagnification. ln order to malte it possibleto use an observationmicroscope with high numerical aperture, the instrument is so devisedthat a powerful observation objective may be placed close to the end ofthe pointer. At very high microscopic magnification, the image of thepointermay be rather blurred, even with high numerical aperture. Inorder to increase the accuracy of the readingvstill further, means areprovided by which it is made possible to make clearly visible thediffraction fringes accompanying the image of the pointer and to readoff the deviation of the pointer by observing the displacement of thesefringes on a scale. i y The condition b) may be deduced from thefollowing reasoning VIf 15 is the deiiection of the coil system, and Athe elastic control (i. e. the restoring moment for unit angulardisplacement) the mean value (t of the amplitudeof the irregularBrownian deflections, according to the equipartition principle, isdetermined by the expression of freedom of the system, said energy atroom rent variation, corresponding to the value of determined'by will be'found tobe in which the function f (a) gives the ratio Ver1.

in which expression R is the total resistance "of the galvanometercircuit, Gr is a quantity fdep'endent on the damping of the movablesystem, and t is the time of indication, i. e.l the time elapsing fromthe closing of cuit until the movable system, within a certain tolerance(in the present case, 1/100() of the final deflection), has reached itsend position. lf a and al are the values of the total damping and theair damping respectively, measured in the critical damp-ing as unit, thefactor G in (2) may be written I @zal t/T between the time of indicationt and the period of oscillation'l (in undamped con-y dition).

As may be seen from (2), the obtainable current sensitivity, which isobviously inversely proportional to z', is proportional to On the otherhand, the factor G is also of considerable importance for the result,and

sliould'be made as small as possible. The

curves in the accompanyingdiagram illustrate the relation between thefactor G and the value of a at three different values of al. As may beseen from the curves, the smallest value of G fory small values of alcorresponds to a value Vof a slightly less than unity, while, for anyvalue of al, a .somewhat higher but still small value of G may beobtained by making a great and considerably greater4 than This isobviously just the condition (t) which should be satisfied at least whenthe external resistance of the coil circuit is small, i. e. when theinstrument is short-circuited. As will be seen, the condition a1 a whichis included in stands in acertaincontradiction to the condition (a), asthe limitation of the air damping will obviously limit the pointerlength, while, on the other hand, a great pointer length is of coursefavorable for Vreading small deflections. It will accordingly benecessary to compromise between the conditions (a) and (b) in regard tothe pointer length. It will be explained below inV what way this may bedone to the best advantage.

The condition (o), according to which the instrument should besubstantially free from Vexternal disturbances, is Vcomplied with bymaking the coil exactly symmetrical with respect to the rotary axis andsuspending it the cirwith such smalldimensions, and in lsuch ay shape,that it can be placed on the stage -of Aan `vordinary lmicroscope, thefordinary con- Vdenser of the microscope A.b eingthen preferablyreplaced by a weal microscope objective. Designing the instrument insaid manlnerhas the vadvantage that the optical equipment I and theadjustment devices of an existing microscope may be used in connectionwith the use of the galvanometer.

YA further suitable arrangement which is useful, particularly inconnection with very delicate coils, consists in providing they coilwith a connecting piece in the form of a thin rodextencing in thedirection of the axis of symmetry (rotary axis), which rod connects` Ytwo diametrically opposite points ofthe coil and serves as a stilfeningspine of the coil. lThis spine facilitates an-exactattachment of thesuspension wires (torsional wires). lt

.takesup the tensile stress of these wires so that said stress doesfnotdeform the delicate windings ofthe coil. Furthermore, the said spine isadapted to beengaged by the damping means of the instrument when thecoil is to be arrested.

The invention will be more scribed with reference to the accompanyingdrawings.` Figure lis a verticalv section on the line l-l in Figure 2 ofa moving coil galvanometer designed according to the invention andadapted to be mounted'on vthe stageof an ordinary microscope. Figure 2shows the same instrument partially in plan view and partially insection on the line 2-2 in Figure l. Figure 8 is a vertical section onthe line 3-3 in Figure l. Figure 4 is a side elevation of the instrumentas viewed In the embodiment according to Figures l `to 5 the moving coilsystem is disposed in a rectangular casing 1 the bottom side of whichextends into the bearing plate 2 adapted to rest on the stage in anordinary microscope and .provided with a circular aperture 3 the centreof which co-incides with the axisiof the microscope at thecorrectpositioning ofthe instrument on the microscope stage.

The moving system of the instrument consists of a circular coil 4Lsuspended between two fine torsional wires, stretched by means ofsprings 5 fixed at projections 6. The coil can turn in usual mannerbetween the pole faces 7 of a permanent horse shoe magnet 8 enclosingthe rest of the instrument which 3 pole faces project inwards throughopposite sides of the casing. The coil14 surrounds an annular core 9 ofsoft iron supported by a collar 10 which projects into the casing from aplate 11 disposed inside the supporting plate 1 forming a side wall ofthe casing. In order to prevent deformations of the light coil and tofacilitate tliearresting, two opposite pointsof the coil co-incidingwith the axis of rotation are united by means of a stiffening connection12, consisting of a rod or tube, for which axial bores'13 are disposedin 'the annular iron core 9.

In the inner cavity of the annular iron core 9 an arresting device forthe coil is disposed and adapted to engage the rod 12.'

Said arresting device consists of two holding clamps 14, preferablyprovided with a coating of cork or the like and adapted to approach orto move away from the rod 12 symmetrically.

The holding clamps consistof angular pieces the feet 15 of which areslideably disposed.

in horizontal slots in theplate 11, and are, by

means of helical springs 16 (Fig. 5) brought to bear against a conicalmember 17 disposed on the arresting screw 18. Said screw extendssomewhat eccentrically through a bushing 19 which is rotatablyjournalled'in a disk 20 secured to plate 1. The outer end of the bushing19 is provided with a threading carrying a stop nut 21.

On the drawings the clamps are shown in cal member 17 ismoved outward byturning the screw 18 the clamps then ybeing pulled together andsqueezing therod 12. i

On accountV of the delicacyA of the movable system it is necessary thatthe holding clamps engage therod parallelly and simultaneously. 'Toadjust the clamps to be parallel with the rod 12 the circular plate 2Ois adapted to turna small angle after the screws v2O `(Fig. 4) have beenloosened.

The symmetrical adjustment of the4 holding clamps to'equal distancesfrom the, rod 12 takes place by turning the bushing 19 after looseningthe stop nut 21. After the adjustment ofthe arresting device has beencarried out in this manner the coil may safely be engaged by said devicewithout a previous slackening of the suspension string.

The `coil 4 carries at its lower portion a pointer 22 consisting e. `g.of a line Vquartzoi glass wire. Said pointer projects into a flat box orpocket 23 extending from the casing 1. In the bottom side of the pocketa glass window 24 is disposed to illuminate the pointer. The upper sideof the box 23 consists of a detachable lid into which a cap 25 turnedupside down istapped which cap at its bottom is provided with "a thinglass window 26 disposed closely above the endof the pointerI oppositethe window 24. The cap 25 is intended toV accommodate the ,observationob.-

jective. By this disposition the observation Objective may approach thepoillferWhlG the' interiorbf the galvanometer remains closed so that nodisturbing aircurre-nts may penetrateltheleto. 1 Y i `The samegend canalternatively alsobe attained without the .concave cap426, in case themicroscope objective is introduced into the interior of the instrumentthrough an aper- 'ture in the topof thebox, the objective being thenenclosed by a soft; packing, byway of Y example a rubber,V packing, atthe` point of llllTOJICBOII.V ,1 i Y' y g Y `*The lid ofthe projectingflat box 23'carries in-the, shownl embodiment of the `galvanometerV alsoltwo insulatedsmall metal plates` 27, (Figa 1) which normally are not inuse,`

and are thencoupled` to the instrument casing butwhich lin certainresearches may serve -toxreduceithe directional force of the movablesystem by means ofyelectrostatic charging i. e. to astatize theinstrument. The indicater 22' should then be electrically conducting. 1

Although of an instrument which jcan be mounted in an ordinarymicroscope brings certain important advantages it may belsuitable inorder to .obtain the utmostpermanency of thezero point,tomount themicroscope objective on the galvanometer proper as is'dia-grammati-`cally shown in Figure 6.` In contradistinction to the above describedembodiment in Y separated position. Atthe'arresting the conithe abovedescribed arrangementwhich the optic axis is parallel with the axis Y ofrotation of the coil system the optic axis is accordingto the embodimentin Figure 61 perpendicular to the axis of rotation and parallel with the`pointer 22.` The latter is ibo providedvwith an extension 28 bent at aright angle which projects downward `between an observation objective29disposed in a light aperture inthe instrument casingrl and-anillumination objective -30 also disposed Y close its to the pointer.-The light from a source of y the casing, which aperture is disposedlright opposite the light aperture of the ,observationy objective. Thearrangement according'M i to Figure, Gvbrings the advantage thatvariations in the temperature Vand therefrom resulting`thermicexpansions only have the smallest possibleinfluence'- upon theposition of thefobjective or the objectives in relation ,l

to the pointer, lin such a way that-evenat a very high magniiication thezero position of the galvanometer and the accuracy oflthe` imageare'substantially variable. Y

Said arrangement has furthermorethe advantage that a central diaphragm'maj7 be constructively embodied with the observation 'i' objective inysuch a manner that said `diaphragm may be disposed on or removed fromthe objective from the outsideV withoutopening the instrument.j Such aneasily shiftable arrangement of a central diaphragm in an instrument ofthe present kindbrings' certain advantages asWill'be described inthefollowing. In'the case of a high microscopic magni- 'icationthereading-oif of the deflection is.

y preferablyrcarriedout by means ofthe difistration in caseV fractionfringes accompanying lthe blurred image of the indicator. f At a givenvalue of the microscopic: magnification .the lWidth of the fringes aresmallest i.v e. the accuracy in the readings-off highest, ifonly certainedge portions of the objective cooperate to produce the image. YThis isattained-by lusing a ce-n- 4 tral diaphragm." Especially in the photo-`rY graphic registration of small deiiections said lcentral diaphragmis`of importance to obtain `the utmost sharpness of the fringes The'diffraction' fringes obtainable Without centraldiaphragm are not quiteso sharp but adequate for subjective reading-off. They liancy, be ofadvantage in photographic regtinctness is not required.' Y

TheV dimensioning yof the movable system l 'y "inclusive of the pointerWill be of special importance if the object in .View is to obtain thehighest possible degree of sensitivity inthe galvanometer;V Itispreviously known (see Phil'. Mag; vol-I, April 1926, page 827) thatthere isa ,certain limit system. In order to Abe able to reach saidlimit Without" evacuating the instrument, yv'vhich would involve greatvrdn'liculties both forY the manufactureand the operation of theinstrument, one must follow certain rules st atedbelow in dimensioningthe instrument. The damping by air frictionacting .upon the lmovablesystem should be less than the electromagnetic damping at closedcircuit,and ,should preferably 'only deliver a smaller portion of thecriticaldamping i. e. the damping required to renderthe motionv of thesystem only just aperiodici f To dimension the movable system lof thegalyanometer in such a manner that said .lim-

it of sensitivity is reached the following dimensioning rules have to befollowed.

The Aair "friction/al resistance `against anarrow cylinder beingdisplaced .transversely .With a velocity u creates a 'force. counteract-`ing the displacement Which per unit length amounts to plu Where 201 isa certaincoe'licient Vrepresenting the; influence of the airvisccsi-vAty. This results in that the frictional vino-V ment Q exerted u1 on astraight ointer ro- 1 .jecting perpendicularly Vto the axis of rota.-

.tion and havingtheylength a cm. can be calculatedwith a gooddegree-ofapproximation from the formula may also, on account of their higherbrilv the highest attainable dis- Y for the sensitivity, `depending'uponthe erratic Brownian fluc- 35 tuation's inthepcs-ition of thegalvanometer in ulichrexpeeoft is the4 angular Velocity y( p=vtheangularvdeiiection in radians). ln similar manner the frictional momentagainst the coil can be approiiimatelyjcalculated.y If it is desiredthat the `quantity Q calculatedfrom4 should constitute'the fraction l/m(by Way of eX;

ample l) of the criticalvdamping' moment (relating to thev a-periodiclimit) and the di-V rectional force is set equal to (in CGS units), thetotal moment of inertia equal to Kv and the period of oscillation'reduced to the undamped condition equal to T, lone has If the value2.1014'ergs of the' molecular' energy -at roomtemperature is'inserted inEquation (l) the followingk expression ,is ob# tained, A I

, c i@ .Y ln order thatthese small fluctuations may be renderedvisibleon the reading-olf scale,

a very high microscope-magni-icaticn n must y be chosen, preferably suchthat the magnified average Brownian deflection is of the order of 6' ofa division on the scale. The numberr n that isl the number of divisionson the scale corresponding to a displacement of one millinieterl in theobject space of course represents here the useful magnification.

power of the microscope that jof a scale di- In other Words7 themagnification shouldbe so" ychosen in relation to the optical resolvingvision corresponds to a displacement which Y i Y n is yopticallyaccurately discernible. deflec-` tion of one division then correspondsto the turning angle of 1 a-lOn radians. Assuming that the mean Browniandeviation, when magnified, equals T15 of a scale division, one iinds andthus, considering (3),

(II) A=4l0`10n2a2 The Equations (I) and (II) determine now incombination the quantities a and A in relation to the chosen values oft, m and-a i. e. they serve as basis for the dimensioning of thegalvanometer system in lorder to `attain the theoretical limit ofysensitivity of these instruments.

' I claim:

l. In a galvanometer in combination, a

magnetic field system, al moving-coil cooperating therewith, astiifening rod connecting two oppositev points of they coil, elasticsus- .i

, and allowing of small rot-ary movements of the coil about the axis ofsaid rod, clamping means adapted to cooperate with said stifening rod, apointer projecting from the coil, a light passage through the instrumentextending perpendicular to the plane of movement of the pointer end, andmeans for applying microscopic instrumentalities to said light, Ipassage for observing the movements of the pointer. 3. In a galvanometerm combination, a magnetic iield system,'a moving-coil cooperatingtherewith, a stiifening rod connect` ing two opposite points of thecoil, elastic suspending means connected with Asaid rod and allowing arotary movement'of the coil about the axis of said rod, two holdingclamps disposed on opposite sides of said rod, and

' means for adjusting said clamps to be parallel withand symmetricallypositioned Vin relation to the rod so as to engage the rodsimultaneously. i

4. In a galvanometer in combination,y a magnetic field system, amoving-coil cooperating therewith, elastic suspending means carryingsaid coil and allowing of small rotary movements thereof about' acertain position of equilibrium, a short pointer proj ecting from thecoil, a light passage through the instrument extending perpendicular tothe plane of movement of the pointer end, anV observation objectivemounted on the galvanometer and applied toy said light passage, andmeans for adjusting said objective with vrelation to the pointer end.

5. VA galvanometer as claimed in claim 4 characterized in that theobservation objective is provided withV` a central diaphragm which isconstructively embodied with the objective in such a manner that iteasily may be placed in positionon or removed from the objective withoutopening the instrument o changing the objective setting.

6. A coil galvanometer as claimed inclaim` 3 characterized in that theholding clamps are pressed by means of springs against at wedge memberwhich is displaceable between the clamps by meansof a screw extendingexcentrically through a rotatable bushing.

7. In a galvanometer in combination', aV magnetic field system, amoving-coil operatingtherewith, a casing enclosing said moving-coil,elastic suspending means carrying said coil and allowing of small rotarymovements thereof aboutk a certain Vposition of equilibrium, apocketprojecting from the casing, a pointer projecting from the coilinto said pocket, the bottom and lid of the pocket f being'provided withlightv apertures, and

' means for applying microscopic instrumentalities to said lightapertures for observo ing themovements of the pointer.

8. In a galvanometer as claimed in claim 7, a concave adjustable capscrewed into the lid and forming a. recess for accommodating anobservation objective, the lightiaperture being arranged at the bottomof said concave cap. 9. In a galvanometer in combination, a magneticfield system', a movin-g coil, said coil being symmetrical with respectto its rotary axis and co-operating'with said mag-V netic field,suspending torsional wires stretch.- ed in opposite directions from thezcoil and allowing of small rotary movement'sthereof about a certainposition of equilibrium, a-

pointer projecting from the coil, 4and means for applyingy microscopicinstrumentalities for reading of the deliectionof said pointer,

Vthe length-of the pointer bearing a relation to the elastic control andthe moment of'inertia such that therBrownian uctuations: are

perceivable at the microscope magnification j used, and that the dampingmoment setl up by'theair friction is smaller than the electromagnetic'damping-moment. In testimony whereof I affix my signature.

' GUSTAF ISING.

